Ford Fiesta mileage has fallen off a cliff

Username03725

Jonathan_Powell said:

Okay, so finally got round to measuring this stuff out somewhat. First of all, I realised I'm actually doing more miles a day than I realised. Thought I was doing a max 10 but when I put my back and forth trips (dropping wife to the station, son to school, daughters to nursery and picking them all back up again) it turns out I'm doing 23 miles a day! So.... 

Full tank - 77,373
3 quarters - 77,428
Half tank - 77,508
Near empty (showing 25 miles left) - 77,612

In total, got about 239 miles. According to Parkers website, the range on this is 480 miles but I don't think that right as I used to drive up north every month 5 or so years ago, about 250 miles, and done it on one tank of petrol. 

There's a world of difference between driving a long way in one trip, presumably on a motorway or at least a major trunk road, and making a succession of short trips one after the other in town.

Manufacturers provide the range as an absolute best-case figure based on their fantasy mpg figures that virtually no-one in the real world will ever get close to, so you may as well forget the range as something that you should be achieving. If your core mileage is a sequence of drop offs etc you're not going to get close to 480 miles on a tank.

Fwiw my own car returns something around 65+ mpg on long m/way trips and around 35 around town. The average is low to high 50s, and is entirely dependent on the mix of driving for any given tankful. I just see the figure and think 'oh it was that this time', rather than expect to hit any specific number. (my financial logging s/w creates the numbers anyway, I don't sit down to work it out just to then dismiss it as irrelevent)

Username03725

scrappy_returns said:

Really easy. Record milage. Put some fuel in. Keep the receipt. Drive. Put some more fuel in when the guage gets to the same point. Compare current milage to orevious milage. Work out how many miles driven for how much fuel purchased and used.

This is the answer. I used to work 20 miles away, with fuel available round the corner from both home and office and at exactly halfway so I was never more than 5 miles from fuel, which meant filling was very consistent - just wait for the fuel light to ping on and use that as the trigger to put a load in. That way it doesn't matter how much goes in as the amount used is pretty consistent and provided a good base for mpg calculations. These days refills are a bit hit & miss so my s/w can report that it's done 12.3mpg for one fill and 96.1 for the next. Overtime it averages out to give reliable figures. Which I then ignore anyway.

Richard53

Brim to brim* over several tankfuls is the only way to get a really reliable fuel consumption figure. Anything else is guesswork. Fuel lights and fuel gauges aren't nearly accurate enough to give you a figure that would be meaningful in diagnosing a problem with the car.

*Brimming the tank is a bad idea. Use the automatic shut-off of the pump instead, first click. Almost as good.